1. Field of the Invention
The present invention relates to a detecting apparatus for detecting moisture content of a stack of sheet media, such as papers, by using electromagnetic waves (hereinafter also referred to as EMW or EM waves), and an image forming apparatus using the detecting apparatus. Particularly, the present invention relates to a detecting apparatus for detecting moisture content of at least a portion of a sheet media stack, by using EM waves at frequency or frequencies including at least a portion of a frequency range from 30 GHz to 30 THz, and an image forming apparatus, such as a printing apparatus, using the detecting apparatus. The EM waves including at least a component in the above frequency range are called terahertz waves in this specification.
2. Description of the Related Background Art
Conventionally, an image forming apparatus, such as a printing apparatus, is likely to be influenced by a change in ambient conditions, such as temperature and humidity. Accordingly, to obtain a stable quality of image under various conditions, a sensor, such as a thermometer or a hygrometer, is disposed around an image forming unit of the apparatus. Information detected by the sensor is fed back to an image forming unit to adjust image forming settings thereof.
In the above method, however, it cannot be said that the sensor detects temperature and moisture with sufficient accuracy. Therefore, it is likely that the image forming unit is inappropriately adjusted.
To cope with the above disadvantage, the following method is proposed (see Japanese Patent Application Laid-Open No. 8(1996)-137145). A method proposed therein uses a detecting apparatus that includes plural light emitting units for emitting light at different frequencies, and a light receiving unit for receiving light emitted from each light emitting unit. The light emitting units are sequentially excited to emit different light, and each light reflected off a measurement object, such as sheet paper, is received by the light receiving unit. Moisture content of the measurement object is estimated based on a comparison result of those received light signals, and image forming settings are adjusted according to the estimated moisture content.
Quite often, however, distribution of moisture content of a paper stack set in a paper holder or tray is not uniform along a paper stacking direction. Moisture contents at upper, central and lower portions of the paper stack may differ from each other. Further, there are also cases where moisture content varies along an in-plane direction of the paper. Hence, moisture content detected by the above method may be that at an upper portion of the paper stack, or that at a portion illuminated with light. Accordingly, especially in a case where printing is performed by a high-speed printer, or other cases where moisture content is required to be detected with greater accuracy, a difference may occur between the measured moisture content of the detected paper and actual moisture content of paper to be printed.
Further, the following methods are also proposed (see Japanese Patent Application Laid-Open No. 8(1996)-334942, and U.S. Patent Application No. 2003/0091351). In those methods, a side of a paper stack is irradiated with light, and paper information is detected. However, the method of the former reference is simply adapted to detect the presence and absence, or quantity of, remaining papers in a paper tray, and the method of the latter reference is only adapted to detect paper thickness. In other words, those methods are not directed to detection of moisture content of paper.